Heat the oven to 350° F.
Put the hollowed peppers on a parchment lined baking sheet and bake for 10 minutes to soften.
Brown the ground beef with salt, pepper, chopped green pepper tops, and onions.
When the beef is browned and the peppers and onions are soft, add the liquid aminos and mushrooms and cook until mushrooms are slightly cooked.
Put a slice of cheese into the bottom of the peppers, fill with ground beef mixture, and top with another slice of cheese.
Cook in the oven until cheese is melted and bubbly, ~10-15 more minutes, checking frequently.
If you have leftover cheesesteak mixture, you can bake it in an oven-safe dish with cheese over top.